Wind turbines are becoming an important source of electric energy. The majority of wind turbines are connected to the electric grid. As the number of larger turbines increases the need for reliable local wind measurements for power curve determination and for advanced control becomes more important.
Having a history of millennia and a century of obscurity following the introduction of fossil fuel engines, the wind power technology had its revival in the mid-seventies as a consequence of the oil crises. Various traditional applications such as water pumping and grain-grinding and small wind turbines for charging of batteries in remote areas have, in modernised form had their renaissance. However, a large fraction of the global energy supply is now delivered in the form of electricity and thus the major effort in wind power technology was placed in the development of electricity generating machines. Therefore, nearly the entire manufactured wind turbine capacity is now so-called grid connected wind turbines.
The modern "stock-average" wind turbine has an aerodynamic rotor with two or--most frequently--three blades. The rotational speed of the rotor is one-half to one revolution per second; through a gearbox the speed is increased to a level suitable to an electrical generator which is typically of the induction type: the rotor of the (electric) generator is magnetised by the grid and once the connection to the grid has been established the rotational speed is kept constant within 1-2% over the range of operational wind speeds, 3-25 m/s. The nacelle (machine cabin) with the gearbox and electric generator is mounted on a tubular or lattice tower through a motor-driven yaw system which by means of a wind direction sensor keeps the wind turbine facing the wind. When the blades are fixed to the rotor hub the machine is said to be stall-regulated because the blade profile angle to the wind has been selected so that the blades stall and loose power when the rated power of the electric generator is reached. If the blades can be pitched the machine is said to be pitch-regulated: when rated power of the electric generator is reached, the blades are pitched to limit the aerodynamic power.
Wind turbines operating with variable speed are also known.
The grid connected wind turbine is equipped with a control system providing smooth cut-in and cut-out of the grid, correct yaw positioning of the rotor, and pitch setting of the blades, stopping of the machine if voltage is too high or too low or if loads on the phases are not balanced, stopping of the machine if unacceptable vibrations are experienced or the temperature of gearbox or generator is too high etc. A number of sensors are connected to the control system; sensors monitoring the air flow may be a wind vane and a cup anemometer mounted on top of the nacelle--just behind the rotor and the flow is therefore rather disturbed compared to free-flow conditions.
The grid-connected wind turbines built and sold commercially in the seventies had installed capacities of 20 to 50 kW and aerodynamic rotor diameters of 5 to 15 m. In 1997, the typical commercially available machines have installed capacities of 500 to 800 kW and rotor diameters of 40 to 50 m. The next generation of machines presently being tested are 1 to 1.5 MW. This urge to increase size probably stems from belief in "economy of scale", i.e. the larger the cheaper, and from the fact that it is easier to fit into the landscape one 1 MW machine than ten 100 kW machines.
Cost of energy from wind power plants depends foremost on the wind energy resources, i.e. the kinetic energy flowing through the area covered by the wind turbine's rotor (per sec.): ##EQU1##
where A.sub.r is the area swept by the rotor, .rho. is the air density, u is the wind speed and f is the frequency distribution of wind speed. The consequence of the available energy being a function of the cubed wind speed is that if the annual mean wind speed at one site is 20% higher than at another site the available energy is {(1.2).sup.3 -1}.congruent.70% higher. Thus, moving from a site with an annual mean wind speed of 5 m/s to a site with 6 m/s the cost of energy (potentially) drops 1/1.7.congruent.58% thus, illustrating the vast importance of the wind climate to the economy of a wind power plant.
The other factors are cost of the wind turbines, operation and maintenance and the efficiency of the wind turbines. Over 20 years, improved aerodynamic and structural design tools and production methods have reduced cost of energy--disregarding the wind climate--by 70%. By 1997 the cost of energy at European sites with high wind speeds was 0.03-0.04 USD/kWh, and at "medium-good" wind sites 0.05-0.07 USD/kWh. Reduction in cost of energy becomes increasingly difficult. However, it is expected that the next 30 years will bring a further reduction of 20-30%.